Device for measuring a bending load

ABSTRACT

A device is provided for measuring a bending load in mechanical constructions, which device enables measurement over a broad range of stresses with high precision and which simultaneously compensates for deviations caused by temperature fluctuations.

The present invention concerns a device for measuring a bending load inconstructions, according to the introductory part of claim 1.

BACKGROUND OF THE INVENTION

The present invention is based upon the principle of using a fibre opticBragg grating. A Bragg grating is a single mode fibre having permanentperiodical variation in the refraction index over a fibre length of forexample 0,1 to 10 cm. Variation in the refraction index is establishedby illuminating the fibre with an UV-laser. A bragg grating reflectslight at a wave length given by the refraction index and the periodrelated to space for the variation in refraction index (grating period),while light outside this wave period will pass the grating more or lessunhindered. The light reflected by the Bragg grating will give a wavelength which varies as a function of a measuring dimension changing therefraction index of the fibre material in the grating and/or by thefibre length in the grating zone (the grating period). Tension in thefibre or temperature will thus give a change in wavelength for the lightreflected in the Bragg grating.

In practical use, temperature can be measured in the range −100° C. to+250° C. at approximately 20 points along a fibre having a length of50-100 km. Using various multiplexing techniques, the number ofmeasurement points can be increased. Examples of areas of applicationare temperature surveillance of power cables, pipelines, electricaltransformers, engines and temperature monitoring of industrialprocesses.

A number of devices for measurement of bending in mechanicalconstructions exist. For special purposes where there is little spaceavailable or there is high temperature, high tension and so forth, allknown devices for measurement of bending load have functionaldisadvantages. For example, measuring bending under water is made withbending sensitive sensors based on electrical elements, which in suchenvironments exhibit low reliability. For other areas of applicationthere may be little space available for installing extra components,such as bend sensors based on electrical induction or capacity (typicaldiameter 10-20 mm). Another problem with sensors based upon electricaleffects is electrically induced noise. For example lightning strikeshave sometimes rendered sensor elements or electronic circuits passive,and thus disabled the bend monitoring.

Accordingly, there is a need for a bend sensor with mainly passivecomponents, that can be utilized in difficult environments andrestrictive spaces.

The objective of the present invention is to provide a device formeasuring bending in and on mechanical constructions.

SUMMARY OF THE INVENTION

The objective is achieved with a device according to the characterizingpart of Claim 1. Further features are disclosed in the dependent claims.

The invention relates to a device for measuring bending in mechanicalconstructions, the device comprising:

a first sensor for measuring a bending load, said sensor including ahousing connected to an optical fibre mounted/prestressed in a firstanchoring point and a second anchoring point by the housing, wherein thefibre is provided with a first Bragg grating, located in the prestressedfibre, the housing being arranged so that the fibre, when exposed to astrong bending force on the housing, is not brought in contact therewithand is exposed to tension load,

a second sensor for measuring tension load, said sensor including ahousing connected to an optical fibre prestressed in a first anchoringpoint and a second anchoring point by the housing, wherein the fibre isprovided with a second Bragg grating, located in the mounted/prestressedfibre, the housing being arranged with an element forcing the fibre tobent along with the housing,

a third sensor for measuring temperature, said sensor including ahousing attached to an optical fibre suspended in a first anchoringpoint and a second anchoring point by the housing without prestressingthe fibre, wherein the fibre includes a third Bragg-grating located inthe suspended fibre, the housing being arranged so that the fibre, by astrong bending or tension of the housing, is not brought into contacttherewith and is exposed to tension load,

wherein one or more of the respective sensors are arranged sequenteallyin the housing along the common optical fibre.

The Bragg-grating in the first sensor will be influenced by bending,tension and thermal effects. The Bragg-grating in the second sensor willmainly is only influenced by tension and thermal effects (for exampleexpansion in the second sensor), while the Bragg-grating in the thirdsensor only is influenced by thermal effects. By combining two or threeof the sensors described above, effects from tension and temperature arecompensated so that the device measures correct bending forces. Allsensors can be integrated into the same housing.

The principal design of a bend measuring sensor according to the presentinvention renders it possible to produce bend measuring sensors withvery small dimensions capable of measuring both small and large bendingradii from distant positions. The device also has the capability ofmeasuring bending in different positions along the same optical fibre.

Examples of mechanical constructions are constructions where the presentinvention is applicable include bridges, flexible pipes and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention is described in further detail withreference to a preferred embodiment illustrated by the accompanyingdrawings, where:

FIG. 1 shows an axial cross-section view of a first sensor for use in adevice according to the present invention for monitoring bending inmechanical structures,

FIG. 1a shows the device from FIG. 1 during bending,

FIG. 2 shows an axial cross-section view of a second sensor for use in adevice according to the present invention for monitoring bending inmechanical structures.

FIG. 2a shows the device from FIG. 2 during bending,

FIG. 3 shows an axial cross-section view of a third sensor for use in adevice according to the present invention for monitoring bending inmechanical structures,

FIG. 3a shows the device from FIG. 3 during bending,

FIG. 4 shows an alternative embodiment of the the device illustrated inFIG. 3,

FIG. 4a shows the device from FIG. 4 during bending, and

FIG. 5 shows an example of how a bending sensor based upon acompensation of effects from tension and temperature can be produced.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a first embodiment of a first sensor according to thepresent invention. This first sensor includes a generally cylindricalhousing 101 with an inner cylindrical bore 102. Inside the bore isarranged an optical fibre 120 prestressed between a first and secondanchoring point 110, 111. A first Bragg grating 121, which in reality isintegrated into the optical fibre 120, is in the figure, for ease ofreference, indicated as a hatched rectangle, located on the freelyprestressed fibre section. FIG. 1a shows the bending sensor and locationof the fibre 120 and grating 121 when the housing 101 is being stronglybent. The bore 102 has an inner diameter which is sufficiently large forthe grating so that it is not exposed to tension load during strongbending. The length and diameter of the housing will vary with theparticular area of use.

During bending, the Bragg grating 121 will be relaxed, where as thelight reflected from the Bragg grating 121 will be influenced bybending, tension and thermal effects from the surrounding atmosphere andfrom the next construction, which is to be measured.

FIG. 2 shows an embodiment of a second sensor for use in a deviceaccording to the present invention, and comprises a generallycylindrical housing 103 having an inner cylindrical bore 104. As isapparent from FIG. 5, the housing 103 can, for the second sensor, beattached to the first housing 101 at the anchoring point 111. Inside thebore 104 in the second sensor is located an optical fibre 120prestressed between the anchoring points 111 and 112. However, the bore104 has an inner diameter slightly larger then the external diameter ofthe fibre 120, which then forces the fibre 120 with the grating 122 tofollow the axis in the housing 103 in bending the same. FIG. 2a showsthis second sensor during strong bending.

During bending of the second sensor, the Bragg grating will be curved instep with the housing 103, and thus not relaxed as a result of thebending. Light reflected from this Bragg grating 122 will then only beinfluenced by tension and thermal effects from the surroundingatmosphere and from the next construction which is to be measured.

FIG. 3 shows an embodiment of a third sensor for use in a deviceaccording to the present invention. The third sensor comprises agenerally cylindrical housing 105 having an inner cylindrical bore 106.As is apparent from FIG. 5, the housing 105 can be attached to thesecond sensor at anchoring point 112. Inside the bore 106 in the thirdsensor is located an optical fibre 120 mounted unstressed between afirst and second anchoring point 112 and 113 located at respective endsof the housing 105. The third Bragg grating is located in the fibresection which is suspended between the anchoring points 112 and 113. Thebore 106 has (like bore 102 of the first sensor) an inner diameter whichis sufficiently large enough not to expose the grating to a tension loadduring bending that is too large. Since the fibre 120 of the thirdsensor is not prestressed, it will not be relaxed at the initialbending. Length and diameter of the housing will vary with theparticular area of use.

During bending of the third sensor, the Bragg grating 123 in the opticalfibre section that is not prestressed, will only to a small degree beexposed to load from bending and tension of the housing. However, Bragggrating 123 will be substantially effected from changes in temperaturecaused by the surrounding atmosphere or the next construction which isto be measured. In this way, the device can be compensated fortemperature-caused displacement in the wave length in light reflectedfrom the Bragg grating 123. The device according to present invention iscalibrated at different temperatures in order to achieve a best possibletemperature dependent measurement of wavelength-shift as a function ofbending.

An alternative for avoiding mechanical influence of the grating 123 inthe third sensor used for temperature compensation is to connect it to,for example, a tube 107 by means of a glue joint, as shown in FIG. 4.The tube 107 (not connected to the housing 105) will then isolate thegrating 123 from bending and tension as shown in FIG. 4a. The tube 107can be a metallic tube or alternatively a glass rod with grooves for theoptical fibre.

FIG. 5 shows a device according to the present invention with first,second and third sensors each with different responses to bendingintegrated in the device. The device comprises a longitudinal housing150, for example in the shape of a tube, with the respective sensorsdistributed along the length of the housing 150 and situated next toeach other. The housing 150 accomodated the respective sensors 101, 102and 103. The housing 150 is connected externally to a surroundingmechanical construction (to be measured for bending) over the entirelength (for example by gluing) or at the anchoring points 110, 111, 112and 113.

When the housing 101 is bent, first Bragg grating 121 will shortenbecause the fibre is situated in a straight length between the anchoringpoints 110 and 111. The second Bragg grating 122 in the bore 104 willexperience a substantially smaller change in length because it is forcedto follow the axis of the housing. The third grating will not beinfluenced by bending of the construction.

In this way, the wavelength shift caused by bending, tension andtemperature can be separated. The housing for all sensor means in thisembodiment is carried out as a common tube. The second sensor means iscarried out by locating a cannula tube 103 with support rings 131 and132 along the axis. The fibre can also be protected on both sides of thebending sensor with cannula tubes 130 and 131, whichhen consitutes asmall and solid cable.

If the bending influenced housing 101 is fixed at the two points 110 and111, the grating 121 will change the prolongation ε₁ according to thefollowing equation:

ε₁ =L ²/24·R ²

where L is the distance between the fixing points 110 and 111 in thehousing 101, and R is the bending radius for the housing. The secondgrating 122 will not experience the longitudinal force due to bending.

If the housing 101, 102 and 103 is glued to the surrounding constructionover the entire length and is exposed to tension in addition to bending,the relative prolongation of the housings ε_(H) will be the same as inthe surrounding construction. This will give the same contribution toall of the housings.

In one embodiment, the entire device in made of metal, except for, ofcourse, the optical fibre. In applications where the diameter of thebending sensor is especially important, both housing 101, 103, 105 andinner tube 107 are made from cannula tubes. The bending sensor can beconstructed with a housing and tubes diameter of mere millimeters.

From the foregoing description it is evident for a person skilled in theart that the various components do not necessarily have the geometryshowed by the drawings. For example, the member to be exposed to bendingmay have a cross-section that deviates from a circular shape; it may beoval, square etc. The same applies to the other components. The centralissue with the invention is, however, that the member to be exposed tobending shall be able to transmit length change further to the connectedBragg grating.

The invention thus provides a device for measuring bending in mechanicalconstructions, which enables measurement over a broad range of stresses,with high precision and which simultanously compensates for deviationscaused by temperature fluctuations. In addition, the device according tothe present invention can be designed to be very small and can thereforebe installed in places where bending measurement usually has not beenpossible. Another advantage with the device according to the inventionis that the fibre is not exposed to external hydrostatic pressure, andwill therefore exhibit a high reliability. Finally, this design does notrequire pressure tight connections for the fibre.

What is claimed is:
 1. Device for measuring bending loads inconstructions, characterized in that the device includes a housing (150)provided with: a first sensor for measuring a bending load, said sensorincluding a housing (101) connected to an optical fibre (120)mounted/prestressed in a first anchoraging point (110) and a secondanchoraging point (111) by the housing, wherein the fibre (120) isprovided with a first Bragg grating (121), located in the prestressedfibre, and housing (101) being arranged so that the fibre (120), whenexposed to a strong bending force on the housing, is not brought incontact therewith and is exposed to tension load, a second sensor formeasuring tension load, said sensor including a housing (103) connectedto an optical fibre (120) prestressed in a first anchoraging point (111)and a second anchoraging point (112) by the housing, wherein the fibre(120) is provided with a second Bragg grating (122), located in themounted/prestressed fibre, the housing (105) being arranged with anelement (103, 104) forcing the fibre to be bent along with the housing(103), a third sensor for measuring temperature, said sensor including ahousing (105) attached to an optical fibre (120) suspended in a firstanchoring point (112) and a second anchoring point (113) by the housingwithout prestressing the fibre, wherein the fibre (120) includes a thirdBragg-grating (123) located in the suspended fibre, the housing (105)being arranged so that the fibre (120), by a strong bending or tensionof the housing, is not brought into contact therewith and is exposed totension load, wherein one or more of the respective sensors are arrangedsequentially in the housing (150) along the common optical fibre (120).2. Device according to claim 1, characterized in that the housing (101)has a generally cylindrical shape.
 3. Device according to claim 1,characterized in that the anchoring points (101, 103, 105, 150) are gluejoints.
 4. Device according to claim 1, characterized in that theelement (103, 104) in the second sensor is a cannula tube (103) arrangedbetween two anchoring points (111 and 112) so that the bendingproperties is approximately similar in all elements.
 5. Device accordingto claim 4, characterized in that the cannula tube (103) is arranged inthe housing (150) and is supported by one or more ring shaped elements(132, 133) against the inner side of the housing (150).
 6. Deviceaccording to claim 1, characterized in that the third Bragg grating(123) for temperature compensation, located in the third sensor isarranged in a freely suspended tube (107) for reducing influence on thegrating (123) from bending and tension.
 7. Device according to claim 1,characterized in that tubes (130 and 131) are provided connected torespective ends of the housing (150) for carrying the optical fibre(120) and protect the fibre and the interior of the device against thesurroundings.